SYNOPSIS

Students play the role of birds, go out on the
school lawn, and pick up toothpick "stick worms" which
have been previously scattered on the lawn in equal numbers of
green-stained and unstained. "Birds" are chased away
before the "worm population" drops too low. Back in
the classroom, the number of green and non-green "worms"
are compared individually and for the whole class. Discussion
relates the experience to the elements of natural selection.
As presented here, it does not lend itself to demonstrating the
effects of selection over multiple generations.

PRINCIPAL CONCEPT

Traits are usually favored by natural selection
only when they result in more reproductively successful offspring.

ASSOCIATED CONCEPTS

1. One common form taken by natural selection is when individuals
blend with their environment, they tend to avoid being seen and
caught by their predators, so more of the "blenders"
tend to survive, and therefore have the opportunity to reproduce
their kind (which also tend to blend).

2. Students have a chance to see selection and competition
working from the perspective of both the prey and the predator.

3. Students realize that the competition occurring in natural
selection is NOT between the prey and the predator, but rather
between members of the same species: which ones can avoid being
caught, and which of the predators can catch the most prey. Whatever
traits contribute to greater survival tend to be found in the
survivors, and therefore more likely to be more common in the
next generation.

4. Large sample size is more consistent and reflective of
a population than small sample size.

ASSESSABLE OBJECTIVES

Students will....

1. be expected to participate actively.

2. recognize the distinctions expressed in the concepts.

3. recognize why natural selection provides the best explanation
for these data.

4. given similar data, recognize that natural selection provides
the best explanation for a mechanism of change of traits in populations

MATERIALS

About 10000 toothpicks, half of which have been stained green.

Worksheet for each student.

TIME

Worksheet, one page, with description of the scenario, data
tables, and discussion questions. You will probably want to create
your own version, incorporating your own name and whatever alliteration
you can create. For that reason, this worksheet is also included
in the Procedures below, so it can be selected, copied, and pasted
into your word processor file for appropriate modification. PDF
version is also available (see bottom of page).

TEACHING STRATEGY

1. Well in advance of this experience, prepare the colored
toothpicks. Probably the best way is to stain them with a light
green cloth dye (be careful...it DOES stain EVERYTHING!); wear
gloves and apron. As an alternative, try well-thinned green paint,
preferably acrylic paint (to avoid the lingering odor and hazard
of fumes). Buy boxes of toothpicks, and note the number per box.
Round toothpicks are more durable, but they also cost a bit more,
and they may be more hazardous to any barefoot people who might
walk on the lawn sometime after the event (even though you try
to get them all picked up afterwards). Plan on returning all
collected toothpicks before you take the next class out, so figure
that each class will have the same population of worms to select
from.

2. Prepare student worksheets, one per student.

3. It's fun to announce to your students on the previous day
that you will be going on a field trip (always builds anticipation).

4. Just before you take your class out to the "field"
(an area of grass, if possible), arrange for a lab assistant,
other teacher, or one of your students to randomly scatter both
colors of toothpicks evenly around that area.

5. Try to find some old farmer's coveralls, a farmer's hat,
and a mock shotgun or pitchfork. Ideally, if you can have your
assistant take the students out to the "pasture" and
get them started looking for "stick-worms", you can
put on the coveralls and hat, grab the gun, and run out to chase
the birds off the pasture and back to class before they nearly
exhaust the population of stick-worms!

6. Be considerate of your colleagues in classrooms between
yours and the pasture. If they are sensitive to "interruptive
behavior", try to have the students go and come by a route
which will be least disruptive. It's a good idea to talk to the
teachers who would be affected by this, and try to get their
cooperation for "just this one time" (?) of silliness,
pointing out the educational benefits of dramatic involvement
in a physical experience. Be sure to caution students to go and
return with minimal disruption of other classrooms.

7. This lesson probably fits best in your introductory unit
on the nature of evolution, especially where you may be trying
to convey a clear understanding of natural selection, as distinctly
different from Lamarck's ideas of organisms surviving because
the "needed" to, or developed traits "in order
to" survive.

8. Either before or after this lesson, it is useful to focus
on natural variations in a population. There are many such examples;
the "Peanut" lab on this site works very well.

9. It is also important during or after discussion of this
lab experience, to recognize and discuss the many other types
of selective pressures which can operate on populations (e.g.
various breeding behaviors, relative sizes of individuals, length
of legs, variations in timing of activity or development, environmental
impact on availability of food, protection against extremes of
temperature, etc.).

PROCEDURES

1. Hand out the Worksheet for students to read, while you
take roll.

2. Summarize briefly what they will be expected to do, including
any cautions (move quietly past classrooms, etc.), and give them
the signal to be used to start getting their stick-worms.

3. Send your class to the pasture (see "Teaching Strategy"
#5).

4. In your best farmer's garb, run out and chase the birds
back to the classroom.

5. Collect students' data to compile class totals. (The use
of a pre-structured spreadsheet on a class computer can speed
up the data entry and calculations, especially if you want to
include the totals from other classes, from this year and/or
last year's, in order for your students to see the greater consistency
of results from larger sample sizes, vs. greater variability
from small [individual] sample sizes).

6. Give them time to complete the answering of discussion
questions (individually, or, perhaps, in small discussion groups).
If time allows, you can begin some informal discussion of what
the data shows. The Questions can be completed as homework.

7. Next day, you can simply go over the discussion questions,
expanding and adapting as other questions may arise spontaneously.

8. One interesting development sometimes occurs which you
should be ready to handle. There is always the "clever-creative
student" (and sometimes a group of them) who may conspire
to "skew the results" by purposely going after the
green stick-worms, avoiding all or most tan worms. If the data
reveals that this has happened, you can use it as an example
of a peculiar variant in the predator population which
caused them to be unusually attracted to green worms, or they
have a special sensitivity to UV radiation, so they are able
to distinguish between the stick-worm green and the grass green,
so those stick-worms stood out more clearly! You can easily take
the discussion in this direction, and it still illustrates the
points you want to make about natural selection, but focusing
on the predator variants rather than the prey. Conspirators are
foiled again!

ASSESSMENT

Observe participation.

Prepare test in which objectives are addressed.

EXTENSIONS

1. If the season, weather or landscape of your school does
not allow you to do this outside, consider getting several squares
of Astro-Turf, enough so that you can have groups of 4-5 students
(sticky birds) per square (each about 60x60 cm square, or larger,
placed on a table top). Use flat toothpicks, broken in half.
Provide plastic forceps as "beaks", or insist on students
using only their thumb and forefinger to pick up the worms. Have
the curtains closed, and turn off the lights just as you announce
"START". Don't give them much time (maybe 30-60 secs.)
before you call "STOP". Then proceed as with the pasture
data. [When you reach an age where you lack the energy to put
on coveralls, run out to the pasture, and chase them back, and
do this for 5 periods in a row, this variation becomes even more
attractive!].

2. Instead of Astro-Turf, use big squares of "very busy"
wallpaper (floral design, or other), and prepare lots of hole-punches
from stacks of that design, along with equal amounts of hole-punches
made from some solid color, or white, or black. Alternatively,
simply have students or lab assistants cut out large numbers
of tiny polygon chips (triangles, rectangles, etc.) randomly
from the wallpaper scraps, and an equal amount (by weight?) of
plain paper pieces.

3. A somewhat more elaborate, yet similar exercise, focuses
on the selection of birds, based on variations in their beaks.
You will find this very entertaining lesson on this site as:
"Natural Selection for Better Beaks".

5. For an outdoor simulation which uses three generations,
try the "Natural Selection of Bean Hunters" lesson
on this site. This one, in a very simple, straightforward way,
simulates three generations. It also focuses on the natural selection
of both the beans and the bean hunters. Other multi-generation
lessons are referenced from that lesson.

6. One of the most creative multi-generation lessons we've
seen for teaching natural selection is Bebbledwark World, created
by Thomas Atkins (ENSI 92) and Gene Nelson, of Fresno, California.
It's a fairly elaborate classroom simulation, requiring considerable
preparation (initially), and can take 2-3 days to carry out.
If you happen to have an old Apple IIe kicking around, there's
an interactive illustrative program to accompany the lesson.
This lesson, with all instructions and materials, is now available
on the web, at its own website, at Bebbledwark World. (When you click on this,
you will leave the ENSI web site. To return, just click on the
"Back" button of your browser.)

7. Take a look at the handy summary: "Comparing Evolution Mechanisms" near the bottom of the "Introduction to Evolution" page. Darwin's and Lamarck's essential elements are compared, and a few common misconceptions are clarified. Scroll down to download the PDF file of this information.

8. Be sure to read two excellent articles dealing with natural
selection in the Natural History magazine (the 12/97-1/98,
issue). In the first one ("The Paradox of the Visibly Irrelevant"
by Stephen Jay Gould, pp. 12-18, 60-66), the author describes
some recent studies of natural selection, and reflects on their
relevance in the context of geological time. The last two pages
are especially interesting. He also comments on micro-/macro-
evolution and punctuated equilibrium in this context.

Also read the article in the same issue by Jonathan Losos
and Kevin de Queiroz on "Darwin's Lizards" (pp.34-39),
describing the the many species of anole lizards of Jamaica (more
discovered every year), and their diverse adaptations to their
many niches.

9. If at all possible, be sure to have your students experience
the cumulative aspect of natural selection, and how this
creates a high probability for new traits to appear which fit
the current selection constratins, and prevail. Try the Natural Selection: A Cumulative Process lesson.

OTHER RESOURCES

National Academy of Sciences. 1998. Teaching About Evolution
and the Nature of Science. Activity 3: Investigating Natural
Selection. Pages 78-80 (see our Resources page, and click on the link for the first book listed, to see those pages online, or order this very valuable book). This is a well-presented multi-generation
lesson, using petri dishes, zip-type bags, pieces of patterned
fabric, paper dots, graph paper, colored pencils, and forceps.
It also recommends use of computer spreadsheet to facilitate
data handling. See the "What's New" area of this site
for details on getting this book.

William F. McComas, ed. Investigating Evolutionary Biology
in the Laboratory. 1994. NABT. There are several exercises which
provide experiences for students to explore natural selection.

ATTRIBUTION

Some of the ideas in this lesson may have been adapted from
earlier, unacknowledged sources without our knowledge. If the
reader believes this to be the case, please let us know, and
appropriate corrections will be made. Thanks.

1. Original Source: Don Dunton, Fred Fisher, Larry Flammer

2. Modified by: Larry Flammer

3. Reviewed / Edited by: Martin Nickels, Craig Nelson, Jean
Beard:

4. Edited / Revised for website by L. Flammer 2/28/99

The following is a useful Student Handout. If you "select"
it then "copy" it (onto your invisible clipboard),
you can shift to your word processor, open a new file, and "paste"
it onto that page. Later, you can make changes to fit your name
and circumstances.
A KEY follows, for teacher use, or for use by students as a self-check.
Below the KEY is a PDF version of the Student Handout, all pre-formatted,
ready to use.

Name_________________________________ Date___________
Per.____

THE NATURAL SELECTION OF STICK-WORMS, or...
"YOU, TOO, CAN BE SELECTED, NATURALLY!"

PROCEDURE: Today, you are all "sticky-birds",
members of the species Birdus stickus, which, as every
good biologist knows, are great connoisseurs of the common stick-worm,
or Stickit tooya in the family Toothpickidae. These worms
have a remarkable resemblance to the common toothpick in structure.
But make no mistake about it, these skinny sticks of nutrition
are actively sought out by all members of your species. Take
a good look around you... those famished, beady-eyed birds are
your competition. Yes, that's right! It's you or them! Go get
'em, sticky-bird!

You are a migrating bird, and today you are passing over Flammer's
fertile farm on your way north for the Winter. That's right,
NORTH! You're a weird bird. In the pasture, you spot the stick-worms
waiting (sticking around). Lucky for you, whenever a stick-worm
sees a sticky-bird, it freezes in fitful fear. This makes it
very easy for you to pick it up with a minimum of effort. Although
there is only one species of this worm, it is found in two color
forms: tan and green (Stickit tooya domesticus, and Stickit
tooya chloris, respectively).

Farmer Flammer favors these funny, frolicking stick-worms
because they fill the fields with finely filled furrows which
favor the flora and fauna of the fertile farm. So watch out!
Gorge yourself with these delicacies, but be ready to make a
hasty retreat in case farmer Flammer sees you.

When we fly out to the grassy pasture, wait along its side
until a signal is given. Then start picking up stick-worms as
fast as you can. You won't have much time because farmer Flammer
and his friends will chase you nasty birds out of his perfect
pasture. When he does, come flying back to your nest as fast
as your little wings can carry you, and count your stick-worms
by color.

On the work sheet provided below, record your totals and the
class totals, as indicated. Then answer the questions.

Total number caught by you:........._______ (Total worms caught
by class:..._______)

QUESTIONS: (answer on back of this sheet)

1. If the total population of worms not eaten today were allowed
to be unmolested by the nasty sticky-birds, what would be the
results in several years time (assuming the stick-worms were
alive and sexy)?

2. Since a change in environment around living organisms can
act as a stimulus for natural selection to occur faster, DESCRIBE
the environment of the stick-worms and how the environment changed
today.

3. Explain the COMPETITION that was set up today: A) between
the sticky-birds, and B) between the stick-worms, both in terms
of the following: 1) what were they competing for? 2) what survival
advantages did some have? and 3) what weaknesses did other have?

4. Why was a time limit placed on the collecting of the worms?

5. What would be the most probable result of this experiment
if it were repeated after the lawn was allowed to dry up and
turn yellow?

6. How would Lamarck have explained the worms' adaptations
over several years?

7. How would Darwin have explained the worms' adaptations
over several years?

8. Briefly state Darwin's theory of natural selection.

OTHER ITEMS WORTHY OF DISCUSSION:

9. What reasons can you offer for the differences in the total
numbers of worms found?

10. What could cause differences in the ratios (or proportions)
collected?

4. So the birds were forced to get what could be seen most
easily, didn't have time to search for hard-to-see worms, which
would eventually happen as the worm population decreased.

5. Probably more green worms would be found; the natural tan
color would blend better with the dry grass color.

6. Lamarck: More worms would be green because they needed
to be green in order to survive being caught.

7. Darwin: More green worms would survive each year, so they
would produce more of their kind each successive year. Since
green is genetically controlled, their offspring would tend to
be green.

8. Natural selection is a process which requires that all
traits within a population vary. And, within a given environment,
certain traits help survival and/or reproduction better than
others, so the owners of those traits tend to survive and reproduce
more often. Since those traits are already inheritable, their
offspring will tend to have those traits. If the environment
changes, different traits and trait combinations are similarly
favored, so eventually they come to characterize that population
(or, eventually, a new species).

9. - easier to see, - different surfaces, - different worm
sizes, - different numbers of worms out there, - different degrees
of aggressiveness of birds.

10. - different color preferences by the birds, - coincidence,
- different ratios of worms in the grass, - one color easier
to see, so selected more often.

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